> >  Today I'm seeing some different characteristics in the water pressure
> > readings. Used to be that after cooling down I could see 4 or 5 lbs
> pressure
> > for many days and I don't think increasing pressure after start up was tied
> > to rpm. Now pressure drops instantly on shutdown and is closely tied to rpm
> > to max of 15lbs as the engine is reved up. This looks bad to me although
> > temps in flight are fine. What do you think?
> >  Peter
> > 
> ------------------------------------
> Peter,
> Where is your pressure guage in relation to pump inlet/outlet?  The pressure
> that the pump puts out goes up at about the square of the rpm.
>
> How is your overflow bottle rigged? Do you have a thermostat?
> Bill Schertz

Ed, your detailed comments make me feel a little better about things not staying constant. Now I'm wondering if maybe I just didn't notice the pressure increasing as I increased the throttle on other runs.

To answer Bill's questions: There is no thermostat. The rad cap is on a stand pipe coming up from one end of the rad. the other end of the rad has the to and from water pump hoses. Overflow bottle feeds from the rad cap. Pressure gauge is in the pump housing where the original mazda temp sensor sat. So I guess this makes it on the output of the pump.

I'm emabarrassed to say I have not checked for bubbles/odour in the rad cap because it requires taking the top cowl off.(couple of dozen screws. I plan to modify the cowl to make it quicker to get at the engine.) Will do that later today. No external signs of coolant leak, Charlie.

Here's a  picture showing bottle and rad cap. I'll put the other one in a second message.